An absorbent core (water absorbent core) containing a hydrophilic fiber such as pulp and a water absorbing agent as its components is widely used in sanitary materials such as disposable diapers, sanitary napkins, incontinence pads and the like, in order to absorb body fluids. The absorbent core is used to allow the sanitary material such as the disposable diaper, the sanitary napkin, the incontinence pad, and the like to absorb body fluids.
Recently, these sanitary materials are required to have thinner sizes to realize higher convenience. Thus, smaller amount of a hydrophilic fiber whose bulk density is low and a larger amount of a water absorbing agent having a superior water absorbing property and high bulk density are included in the absorbent core so that a ratio of the hydrophilic fiber is decreased and a ratio of the water absorbent resin is decreased. The ratio of the water absorbing agent contained in the absorbent core is increased in this manner, thereby making the sanitary material thinner without dropping properties such as a water absorption capacity.
However, the sanitary material which includes a smaller amount of the hydrophilic fiber and a larger amount of the water absorbing agent is preferable merely in terms of liquid storage, but raises problems in terms of distribution and diffusion of liquid in actual use in diapers or the like. For example, when a large amount of the water absorbing agent is used, the water absorbing agent becomes soft and gelatinous upon absorbing water. This causes a gel blocking phenomenon which prevents liquid from being absorbed in the water absorbing agent. As a result, a liquid diffusing property of the diaper significantly drops. In order to avoid such phenomenon and to keep high absorbing property of the absorbent core, a ratio of the hydrophilic fiber and the water absorbing agent is inevitably limited, so that there is a limit in making the sanitary material thinner.
In order to achieve a sufficient water absorption capacity while preventing the gel blocking, it is necessary to obtain a water absorbing agent which is excellent in balance between an absorbency represented by centrifugal retention capacity (CRC) or the like and liquid permeability represented by saline flow conductivity (SFC) or the like. However, the relation between the two properties is such that the one drops when the other is enhanced. Thus, it is difficult to enhance the relation therebetween up to a sufficient level. As related means, the following techniques are known for example.
Patent Document 1 discloses a water absorbent resin obtained by treatment with trivalent or further polyvalent cation. Patent Document 2 discloses a technique using an electrostatic or three-dimensional spacer with respect to a water absorbent resin. Patent Document 3 discloses a particulate water absorbing agent including (i) water absorbent resin particles obtained by cross-linking surfaces of irregularly-pulverized particles obtained by cross-linking a monomer containing acrylic acid and/or salt thereof and (ii) a liquid permeability improving agent in order to provide a water absorbing agent having both capillary suction and liquid permeability.
Each of the techniques disclosed in Patent Documents 1 to 3 is such that an additive for improving liquid permeability, e.g., a polyvalent metal salt (metal cation), inorganic particles, polycation, and the like, is added to the water absorbent resin so as to improve the relation between the absorbency and the liquid permeability. But this technique fails to realize the favorable relation (balance). Thus, such technique has been required to be further improved.
Further, the water absorbing agent including the water absorbent resin particles as a main component has a moisture absorbing property, so that its fluidity significantly drops under high humidity. This raises such problem that the workability drops and a similar problem. As means for improving the problem, the following techniques are known.
Patent Document 4 discloses an anti-caking composition including (a) water-insoluble polymer particles which absorb water and are slightly cross-linked, (b) an effective amount of anti-caking agent, and (c) a hydrophobic dust controlling agent of 100 ppm to 6000 ppm relative to a weight of the polymer particles.
As sanitary material water absorbent resin particles whose absorption rate and moisture absorption blocking property are improved, Patent Document 5 discloses modified sanitary material water absorbent resin particles which are a cross-linked polymer of an ethylenic unsaturated monomer having acrylic acid and/or salt thereof as a main component and which have been treated with organic polysiloxane in a liquid state at room temperature.
As water absorbent resin particles whose absorption rate, moisture absorption blocking property, and dust generating property are improved, Patent Document 6 discloses modified water absorbent resin particles obtained by treating particles of a water absorbent resin with a silicone surfactant.
Patent Document 7 discloses a particle material composition which contains inorganic powder (kaoline clay) blended with high water absorbent polymer particles and which is characterized in that less than about 60 mass % of the polymer particles pass through a U.S. standard 50 mesh sieve (300 μm in mesh size).
Patent Document 8 discloses a swelling polymer which is coated with nonionic nitrous surfactant and, as desired, Lewis acid, and which is heat-treated.
However, the foregoing conventional techniques fail to provide a water absorbing agent which satisfactorily achieves various properties.
Further, Patent Document 9 discloses a water absorbent resin which is cross-linked by a polyhydroxy compound and which is coated with or bonds to a nonionic surfactant whose HLB ranges from 3 to 10.
Further, Patent Document 10 discloses a super-absorbent material to which a surfactant has been applied and has permanent wettability.
Further, Patent Document 11 discloses an arrangement in which a surfactant aqueous solution or a surfactant aqueous dispersion liquid is sprayed onto hydrogel formation polymer so as to give specific properties.
Also the techniques disclosed in Patent Documents 9 to 11 fail to realize the favorable relation (balance), so that these techniques are required to be further improved.
[Patent Document 1]
    International Publication No. 2001/74913 pamphlet (Publication date: Oct. 11, 2001)[Patent Document 2]    U.S. Patent application No. 2002/0128618, (Publication date: Sep. 12, 2002)[Patent Document 3]    International Publication No. 2004/069915 pamphlet (Publication date: Aug. 19, 2004)[Patent Document 4]    International Publication No. 97/037695 pamphlet (Publication date: Oct. 16, 1997)[Patent Document 5]    Japanese Patent No. 3169133, (Publication date: May 21, 2001)[Patent Document 6]    Japanese Unexamined Patent Publication No. 136966/1997 (Tokukaihei 9-136966) (Publication date: May 27, 1997)[Patent Document 7]    International Publication No. 2000/010619 pamphlet (Publication Date: Mar. 2, 2003)[Patent Document 8]    International Publication No. 2002/034384 pamphlet (Publication date: May 2, 2002)[Patent Document 9]    Japanese Unexamined Patent Publication No. 156034/1993 (Tokukaihei 5-156034) (Publication date: Jun. 22, 1993)[Patent Document 10]    Japanese Translation of PCT International Application No. 527213/2003 (Tokuhyo 2003-527213) (Publication date: Sep. 16, 2003)[Patent Document 11]    Japanese Translation of PCT International Application No. 511489/2003 (Tokuhyo 2003-511489) (Publication date: Mar. 25, 2003)